Why does Abigail want control over Mary and the other girls in courts?
Abigail is counting on the other girls to help corroborate her stories. As she accuses anyone she chooses, she needs their support because the courts currently believe the girls over the testimony of time-tested moral people. Should she fail to have their support, she would stand alone as one crazy person making random accusations (which is actually what she is doing).
The other girls are also keeping secrets and lies of Abby's. She has threatened them on more than one occasion to stay true to her. For example, in the beginning, although the actually conjured spirits, Abby tells the girls to only claim that they danced.
A very specific example of this control occurs when one girl tries to rebel against Abigail. When Mary Warren begins testifying truthfully, Abigail pressures the other girls into copying Mary Warren. Warren finally caves and joins the other girls so they stop imitating her.